The positive photoresist composition commonly used is a composition comprising an alkali-soluble resin and a. naphthoquinonediazide compound as a photosensitive material. Examples thereof include a "novolak-type phenol resin/naphthoquinonediazide-substituted compound" described in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470 and a "novolak resin comprising cresol-formaldehyde/trihydroxybenzophenone-1,2-naphthoquinonediazide sulfonic acid ester" as a most typical composition, described in L. F. Thompson, Introduction to Microlithography, No. 2. 19, pp. 112-121, ACS Shuppan.
In these positive photoresists fundamentally comprising a novolak resin and a quinonediazide compound, the novolak resin exhibits high resistance against plasma etching and the naphthoquinonediazide compound acts as a dissolution inhibitor. The naphthoquinonediazide generates a carboxylic acid on irradiation of light and loses its dissolution inhibiting ability to thereby elevate the alkali solubility of the novolak resin.
From this viewpoint, a large number of positive photoresists comprising a novolak resin and a naphtho-quinonediazide-base photosensitive material have heretofore been developed and used in practice, and satisfactory effects can be successfully attained in the working for the line width of approximately from 0.8 to 2 .mu.m.
However, integrated circuits are being more and more intensified in the integration degree and the production of a semiconductor substrate such as VLSI requires working of an ultrafine pattern comprising lines having a width of a half micron or less.
According to one of known techniques for achieving miniaturization of a pattern, a resist pattern is formed using an exposure light source having a shorter wavelength. This technique can be described using the following Rayleigh's formula for the resolution R (line width) of an optical system: EQU R=k.multidot..lambda./NA
(wherein .lambda. is a wavelength of the exposure light source, NA is a numerical aperture of the lens and k is a process constant). As known from this formula, a higher resolution, namely, a smaller R value can be obtained by reducing the wavelength .lambda. of the exposure light source.
For example, in the production of a DRAM having an integration degree up to 64 M bits, the i-line (365 nm) of a high-pressure mercury lamp is used at present as the light source. In the mass production process of 256-M bit DRAMs, use of a KrF excimer laser (248 nm) in place of the i-line is being studied. Further, for producing DRAMs having an integration degree of 1 G bits or more, a light source having a further shorter wavelength is being studied. To this effect, an ArF excimer laser (193 nm), an F.sub.2 excimer laser (157 nm), an X ray, an electron beam and the like are considered to be effective (see, Takumi Ueno et al., Tanhacho Photoresist Zairyo -ULSI Ni Muketa Bisai Kako- (Short Wavelength Photoresist Material -Fine Working Toward ULSI-), Bunshin Shuppan (1988).
When a conventional resist comprising a novolak and a naphthoquinonediazide compound is used for the lithography pattern formation using a far ultraviolet ray or excimer laser beam, the novolak and naphthoquinonediazide exhibit strong absorption in the far ultraviolet region and the light scarcely reaches the resist bottom, as a result, only a tapered pattern having low sensitivity can be obtained.
One of the techniques for solving this problem is the chemical amplification-type resist composition described in U.S. Pat. No. 4,491,628 and European Patent No. 249,139. The chemical amplification-type positive resist composition is a pattern formation material which produces an acid in the exposed area on irradiation of radiation such as far ultraviolet ray and due to the reaction using this acid as a catalyst, differentiates the solubility in the developer between the area irradiated with the active radiation and the non-irradiated area to form a pattern on a substrate.
Examples thereof include combinations of a compound capable of generating an acid by the photolysis with an acetal or O,N-acetal compound (see, JP-A-48-89003 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), with an ortho ester or amide acetal compound (JP-A-51-120714), with a polymer having an acetal or ketal group on the main chain (JP-A-53-133429), with an enol ether compound (JP-A-55-12995), with an N-acyliminocarbonic acid compound (JP-A-55-126236), with a polymer having an ortho ester group on the main chain (JP-A-56-17345), with a tertiary alkyl ester compound (JP-A-60-3625), with a silyl ester compound (JP-A-60-10247) or with a silyl ether compound (JP-A-60-37549, JP-A-60-121446). These combinations in principle have a quantum yield exceeding 1 and therefore exhibit high photosensitivity.
A system which decomposes by heating in the presence of an acid and is alkali-solubilized is also used and examples thereof include combination systems of a compound capable of generating an acid on exposure with an ester or carbonic acid ester compound of tertiary or secondary carbon (e.g., t-butyl, 2-cyclohexenyl) described in JP-A-59-45439, JP-A-60-3625, JP-A-62-229242, JP-A-63-27829, JP-A-63-36240, JP-A-63-250642, JP-A-5-181279, Polym. Eng. Sce., Vol. 23, page 1012 (1983), ACS. Sym., Vol. 242, page 11 (1984), Semiconductor World 1987. November, page 91, Macromolecules, Vol. 21, page 1475 (1988), and SPIE, Vol. 920, page 42 (1988), with an acetal compound described in JP-A-4-219757, JP-A-5-249682 and JP-A-6-65332, or with a t-butyl ether compound described in JP-A-4-211258 and JP-A-6-65333.
These systems mainly comprise a resin having a basic skeleton of poly(hydroxystyrene) which is small in the absorption in the region of 248 nm and therefore, when the exposure light source is a KrF excimer laser, they are highly sensitive, highly resolving and capable of forming a good pattern and can be a good system as compared with conventional naphthoquinonediazide/novolak resin systems.
However, when the light source has a further shorter wavelength, for example, when the exposure light source used is an ArF excimer laser (193 nm), the above-described chemical amplification type systems are still deficient because the compound having an aromatic group substantially exhibits large absorption in the region of 193 nm. As the polymer having small absorption in the 193 nm region, J. Vac. Sci. Technol., B9, 3357 (1991) describes the use of poly(meth)acrylate, however, this polymer has a problem that the resistance against dry etching which is commonly performed in the production process of semiconductors is low as compared with conventional phenol resins having an aromatic group.
Proc. of SPIE, 1672, 66 (1922) has reported that polymers having an alicyclic group exhibit a dry etching resistant property on the same level as that of the compound having an aromatic group and at the same time, has small absorption in the 193 nm region, and use of these polymers is being aggressively studied in recent years. Specific examples thereof include the polymers described in JP-A-4-39665, JP-A-5-80515, JP-A-5-265212, JP-A-5-297591, JP-A-5-346668, JP-A-6-289615, JP-A-6-324494, JP-A-7-49568, JP-A-7-185046, JP-A-7-191463, JP-A-7-199467, JP-A-7-234511, JP-A-7-252324 and JP-A-8-259626. The polymers having an alicyclic group described in these publications use a carboxyl group in place of a phenolic OH group as the group of imparting solubility in an alkali developer so as to reduce the absorption in the 193 nm region. However, the carboxyl group has large solubility in a developer and with a developer hitherto used for resist materials (for example, a 2.38% aqueous tetramethylammonium hydroxide solution), even the unexposed area dissolves at the development to cause a problem of film thickness loss.
In order to solve this problem, it is necessary to dilute the developer and reduce the concentration or to reduce the carboxyl group content in the polymer. When the developer concentration is reduced, the reproducibility by the development becomes a problem, and when the carboxyl group content in the polymer is reduced, the polymer is intensified in the hydrophobic property and the adhesion to the substrate is disadvantageously deteriorated.